Calibratable electrical instrument



Dec. 14, 1954 H. OTZMANN, JR 2,697,204

CALIBRATABLE ELECTRICAL INSTRUMENT Filed April 7, 1951 37a 1 37d 37c Fig.3.

INVENTOR Henry Ofzmonn, Jr.

BY I

ATTORNEY United States Patent Office 2,697,204 CALIBRATABLE ELECTRICAL INSTRUMENT Henry Otzmann, Jr., Roselle Park, N. J., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Appiication April 7, 1951, Serial No. 219,817 6 Claims. (Cl. 324-151) This invention relates to electromagnetic instruments, and it has particular relation to permanent-magnet movable-coil electrical measuring instruments.

The invention is suitable for electrical instruments suitable for permanent-magnet movable-coil instruments and will be described with reference to such an instrument.

The strument owing its manufacture.

The calibration of an instrument also may be de endent on the s ecific application of the instrument following its manufacture. For example, a steel panel on which a shunt will a1. Patent 2,179,305.

In accordance with the invention, an auxiliary permanent a magnetic field employed As applied to a permanent-magnet moving-coil instrument, this calibrator is produce an auxiliary magnetomotive force across the airgap of the instrument. The magnitude and polarity of the auxiliary magnetomotive force are adjusted by rotation of the calibrator.

It is, therefore, an object of the 2,597,204 Patented Dec. 14, 19542 Fig. 2 is a view in bottom plan of a spring retainer employed in the instrument of Fig. 1; and

Fig. 3 is a view in elevation of an auxiliary permanent-magnet assembly employed as a calibrator in the instrument of Fig. 1.

Referring to the drawing, magnetic instrument having an airgap within which a magnetic field is produced. This instrument takes the form of a permanent-magnet moving-coil instrument aving a stator including a magnetic structure which definines two airgaps 1 Any portion of Fig. 1 shows an electrothe airgaps 1 and 3. In the specific embodiment of Fig. l, the magnetic structure includes a U-shaped permanent magnet 5 having pole pieces 7 and 9 secured in Q a a cured thereto. The free end of this with a pivot for receptlon in a suitable bearing screw 13 towards a predetermined stator and it also may be employed for establishing an electrical connection between one end of the core 13 and an external circuit. A similar stub shaft, spring and bearing screw assembly mav be provided for the lower end of the coil 13.

The shaft 15 also has secured thereto a pointer 21 which cooperates with a scale 23 to indicate the value of the quantity which is measured by the measuring instr ument.

stator. Such The portions of been specifically described are Since the construction and operainstruments of sary.

In order to adjust the stren th of the magnetic field in the airgaos 1 and 3, an auxiliarv permanent ma net or calibrator 29 is incorporated in the i spouse to dinstment of the calibrator 29.

Convenientlv, the c libra r 29 tween, two po ar extensions 31 and The calibrator 29 is mounte for rotation on the bridge 35 in anv suitable manner. For exam le, the calibrator 29 may be secured to a support 37 (Fig. 3). port has a cylindrical body 37a which has enlarged head 37b at one end and a screwdriver slot 376 at its remaining end. A peripheral groove 37a is formed in the body to define a neck 372 which has a diameter smaller than the may be constructed of a nonmagnetic material. such as brass, and may be secured to the calibrator 29 in any suitablemanner; such as by. soldering.

The bridge 35 (Fig; to receive'snugly the ibody-37a of'the:support. The head 37b oft'thesupport acts as a'spacenbetween the calibrator 29 and the bridge 35. tion by means-of a bow spring 4'1i hi'3h'h3S a'centrallv disposed slot143 formed therein (Fig; 2). is prop rtioned to": receive. snugly the? neck In addition; the'spr'ing has on its convex'side depression 45' having. athediameter of'the body 37a '(Fig. 3). 41" is in mounted position as illustrated in' Fig.- 1, the center of the spring is head 37b into firm engagement with 37. (Fig. 3 a circular the bridge 35;. Be:

cause of the constant pressure e erted'by the spring" on. the calibrator, the calibrator is held securely in mountedositionand yetmay' b'e rotated as'desired. Itwill be notedfurther 'thatthe calibrator may be'readily replaced.

For example, the calibrator of Fig. 1 may be replaced by a calibrator similar in construction ferent coercive force.

Preferably, the calibrator and" the associated casing are so constructed that the calibrator maybe adjusted without disturbing thecasingin any wa Forrexamnle, the base 27 of the casing may-have an: opening 27a formed therein through which prevent entry'of dust into'the casing, the base 27a may be provided with atubular extension 27b which snugly embraces the end of the body 3721 which containsthe screwdriver slot 370.

It is believed that the operatiom of 'the calibrator now may beset f rth: Polarities resented in Fig 1' by theoonventional' polarity 'mark N for north-pole and S'ior' south pole; with the polarities as illustrated inFig; 1", the calibrator 29' assists the permanent magnet 5; in directing magnetic flux through the airgaps' 1' and 3 Consequently; density in the airgapsis greater than be provided'by the permanent magnet 5' alone.

Let it be assumed next that the calibrator 29 is rotated 90 from1theposition-illustrated in Fig. 1'. When so located, the calibrator 29 has negligible effect on the magnetic field's present in the airgaps- 1' and 3*. Consequently, the magnetic substantially to that which would be provided by the ermanent magnet 5: alone.

Should the calibrator 29 be rotated 180 from the position illustrated in-Fig; 1', the poles of'the calibrator would be adjacent respectively, unlike poles of the magnetv 5. Under these. circumstances, the magnetic flux density in the tially less than the densities obtained by provision of the permanent magnet 5" alone. of the calibrator would produce intermediate effects on the" magnetic should be noted that the adjustment for the calibrator 29 is a continuous adjustment. The calibrator can be rotated continuously and does not require steps of any type. Furthermore, it should be noted that the calibrator 29 permits the full strength of the permanent magnet 5 to be utilized. For example, the tolerance prescribed for manufacture ofv the instrument may be such that with the calibrator displaced 90 from the position illustated in Fig. the average instrument measures correctly. This means that the entire strength of the permanent magnet 5 is utilized in the average instrument. Inaccuracies in instruments which deviate from the average may be compensated by suitable adjustment of the calibrator 9. To illustrate the range of adjustment which may be obtained by such a calibrator, a variation of the order of 45% of the full-load reading of the instrument may be obtained readily by adjustment of the calibrator 29. Since the adjustment provided by the calibrator 29 is self-contained, the instrument may be readily adjusted in the field for use on magnetic or non-magnetic panel's, as desired.

Although the invention has been described with refdiameter. of. the. body.. Thesupportv 1) has an .opening proportioned The calibrator is-retained in P051;

diame er: slightly greater than When'the spring biased? downwardly to urge" thebut having a. dif'-- I a screwdriver may be. inserted for the-purpose of adjusting'the'calibrator: To

for the calibrator arev rec the magnetic fiux. that which would.

flux density in the airgaps is equal airgaps 1- and' 3" would be substan- Intermediate positions fiux density on. the airgaps: 1 and 3. It

A erence. to, certain. specific, embodiments thereof, numerous modifications falling within the spirit and the scope of the invention are possible.

1 claim as my invention:

1. In a permanent-magnet, moving-coil instrument, a magnetic structure having an; airgap arcuate about an axis, said magnetic structure including a first permanent magnet for producingamagnetic field inthe airgap, a coil having a coil side disposed in said air'gap, means mounting: thecoil for-rotation relative to the magnetic structure about said axis, said coil side being positioned to develop a' motor action by'the reaction ofcurreut flowing therethrough and said magnetic field which urges the coil about-'saidaxis relative to the magnetic structure, a second permanent magnet co operating with the magneticstructure to shunt said airgap, and means mounting the second permanent magnetfor. adjustmentabout an axis to,vary the ma netomotive force applied thereby across, said airgap saidmotor-action operating-to move the coil relative to thesecond permanent magnet.

2. An instrument as claimed inclaiml' in combinationwith a casing havinga base member, substantially transverse to said axes, said.sec0nd permanent magnet leing. accessible for adjustmentthrough said base mem- 3. An instrument as defined in claim 1 wherein the magnetic structure. includes a pair of spaced polepieces magnetically connected respectively to the poles of said firsbpermanentmagnet, the poles of saidfsecond permanent magnetbeing positionedior movementrespectively towards, and, away from said polepieces in 'response to adjustment of, the second permanent magnet about the secondaxis.

4. An instrument as claimed in claim 3, wherein said mounting; means includes a non-magnetic bridgemember connecting said pole pieces, and means" mounting, the second permanent magnet on said bridge member-- for rotation: relative tov the bridge. member about said. axis; i

5'. In a moving coil' instrumen a magnetic struc ture defining an airgap, said magnetic structure including means for establishing a first magnetomotive force across said' ai'rgap, a permanent" magnet for establishinga second magnetoinotive force across the airgap, a coil having. a coil side in the. airgap; means mounting thecoil for. movement relative to the magnetic struc-f ture and the. permanent magnet in response to there,- action'. between current passin through the coil and a magnetic field in the airgap, and means mounting the permanent magnet for adjustment relative tothe magnetic structure through a range sufli'cient to bring said riliiagnetomotive forces into opposing or aiding relations ips.

6. In a moving coil instrument, amagnetic struc-- ture defining an airgap, said' magnetic structure includi'ng a first permanent magnet for establishing a first magnetomotive force across said ai'rgap, a second permanent magnet for establishing a second magnetornotive force across the airgap, a coil having a coil side in theairgap, means mounting the coil formovement relative to'the magnetic structure and the permanent magnets in. response to the reaction between current passing through the coil and a magnetic field in the airgap, and means mounting" the second. permanent magnet for rotational adjustment relativev to; the magnetic-- structure;

about an axis andthrough a range suificient to bring, said magnetomotive forces. into. opposing: 0r.- aiding re;

lationships.

References Cited. in the file of this patent UNITED STATES.- PATENTS- Number Name Date 497,448 I-loyt May 16', 1893 691,393 La Roche Jan. 21, 1902 1,282,497 Trent Oct. 22;, 1918 1,479,524 Van Guilder Jan. 1, 1924 2,102,409 Fans Dec. 14; 1937 2,538,740 Wallace Jan. 16, 1951 

